Downstream sapphire elbow joint for remote plasma generator

ABSTRACT

A remote plasma generator, coupling microwave frequency energy to a gas and delivering radicals to a downstream process chamber, includes several features which, in conjunction, enable highly efficient radical generation. In the illustrated embodiments, more efficient delivery of oxygen and fluorine radicals translates to more rapid photoresist etch or ash rates. A single-crystal, one-piece sapphire applicator and transport tube minimizes recombination of radicals in route to the process chamber and includes a bend to avoid direct line of sight from the glow discharge to the downstream process chamber. Microwave transparent cooling fluid within a cooling jacket around the applicator enables high power, high temperature plasma production. Additionally, dynamic impedance matching via a sliding short at the terminus of the microwave cavity reduces power loss through reflected energy. At the same time, a low profile microwave trap produces a more dense plasma to increase radical production. In one embodiment, fluorine and oxygen radicals are separately generated and mixed just upstream of the process chamber, enabling individually optimized radical generation of the two species.

REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. Application Ser. No.09/546,750, filed Apr. 11, 2000, which claims the priority benefit under35 U.S.C. §119(e) of provisional application No. 60/128,859, filed Apr.12, 1999.

FIELD OF THE INVENTION

The present invention relates generally to remote plasma generators, forsemiconductor processing equipment, and more particularly to microwavepower plasma generators for ashing or stripping photoresist and removingpolymeric residue from semiconductor substrates.

BACKGROUND OF THE INVENTION

In fabricating integrated circuits, photoresist is coated oversemiconductor substrates and patterned through selective exposure todeveloping light and removal of either the developed or undevelopedportions. The patterned resist forms a mask used to extend the patterninto underlying layers, such as oxides or metal layers, by etchingthrough the holes in the mask. Masks are also commonly used toselectively dope regions of the substrate by ion implantation. Once themask has been employed, it is typically removed by an oxidation process.The oxidizing process is referred to in the industry as resist“stripping” or “ashing.”

Increased throughput is a primary objective in the semiconductorindustry, particularly in the current era of single-wafer processing.Any reduction in the time required for processing each substrateserially in single-wafer processing systems can lead to significant costsavings in a highly competitive industry. In the case of resiststripping, the rate of processing can be increased by supplying reactiveoxygen free radicals to the substrate. For example, dissociation ofoxygen-containing gases, such as diatomic oxygen gas (O₂), results inatomic oxygen (O), also known as oxygen free radicals.

The addition of fluorine, in the form of NF₃, CF₄, SF₆ or fluorine freeradicals (F), often aids the stripping process where resist chemistryhas been complicated by prior processing. For example, it is difficultto remove photoresist that has been subjected to ion implantation, suchas that employed in electrically doping the semiconductor substratethrough the mask. Similarly, reactive ion etch (RIE) through resistmasks, particularly where metal is exposed during the etch tend to formpolymeric residue, which is also difficult to remove by oxidation alone.In each of these situations, application of fluorinated chemistries aidscleaning the resist and residue from the substrate. Fluorine is alsocommonly used in other cleaning or etching steps.

Maximizing the generation of oxygen (and/or fluorine) free radicalspositively influences the rate at which resist can be stripped, thusincreasing substrate throughput. Such free radicals are commonlyproduced by coupling energy from a microwave power source tooxygen-containing gas. Remote microwave plasma generators guidemicrowave energy produced in a magnetron through a waveguide to aresonant cavity or “applicator,” where the energy is coupled to a gasflowing through the cavity. The gas is excited, thereby forming oxygenfree radicals (O). Fluorine free radicals (F) are similarly formed whenfluorine source gas is added to the flow. Common source gases include O₂ for providing O, and NF₃, CF₄, SF₆ or C₂F₆ for providing F. Nitrogen(N₂) or forming gas (N₂/H₂) is often added to the flow to increaseparticle kinetics and thereby improve the efficiency of radicalgeneration.

While microwave radical generators can lead to significant improvementsin ash rates, conventional technology remains somewhat limiting. Theplasma ignited by the microwave power, for example, includes highlyenergetic ions, electrons and free radicals (e.g., O, F, N). While O andF free radicals are desirable for stripping and cleaning resist from thesubstrate, direct contact with other constituents of the plasma candamage the substrate and the process chamber. Additionally, the plasmaemits ultraviolet (UV) radiation, which is also harmful to structures onthe substrate.

Direct contact between the plasma and the process chamber can be avoidedby providing a transport tube between the microwave cavity or applicatorand the process chamber. The length of the tube is selected to encouragerecombination of the more energetic particles along the length of thetube, forming stable, less damaging atoms and compounds. Less reactive Fand O radicals reach the process chamber downstream of the microwaveplasma source in greater proportions than the ions. Because the processchamber is located downstream of the plasma source, this arrangement isknown as a chemical downstream etch (CDE) reactor. By creating a bend inthe tube, the process chamber is kept out of direct line of sight withthe plasma, such that harmful UV radiation from the glow discharge doesnot reach the substrate.

The tube itself, however, places several limitations on the CDE reactor.Conventionally, both the applicator and the transport tube are formed ofquartz. Quartz exhibits advantageously low rates of O and Frecombination, permitting these desired radicals to reach the processchamber while ions generated in the plasma source recombine.Unfortunately, quartz is highly susceptible to fluorine attack. Thus,the quartz transport tube and particularly the quartz applicator, whichis subject to direct contact with the plasma, deteriorates rapidly andmust be frequently replaced. Each replacement of the quartz tubing notonly incurs the cost of the tubing itself, but more importantly leads toreactor downtime during tube replacement, and consequent reduction insubstrate throughput.

An alternative material for applicators and/or transport tubes issapphire (Al₂O₃). While highly resistant to fluorine attack, sapphiretubes have their own shortcomings. For example, sapphire transport tubesexhibit much higher rates of O and F recombination as compared toquartz, resulting in lower ash rates. Additionally, sapphire issusceptible to cracking due to thermal stresses created by the energeticplasma, limiting the power which can be safely employed. Lower plasmapower means less generation of free radicals, which in turn also reducesthe ash rate. While employing single-crystal sapphire somewhat improvesthe strength of the tube relative to polycrystalline sapphire, safepower levels for single-crystal sapphire are still low compared to thosewhich can be employed for quartz tubes. Moreover, bonding material atthe joint between sapphire sections that create the bend in thetransport tube, which prevents UV radiation from reaching the processchamber, is typically as susceptible to fluorine ion attack as isquartz.

Other limitations on the production of radicals in a conventionalmicrowave plasma generator relate to the efficiency of the energycoupling mechanism. Much of the microwave power supplied by themagnetron is lost in power reflected back up the waveguide, where it isabsorbed by an isolator module designed to protect the magnetron.

Energy also escapes where the applicator carries source gas in and freeradicals out of the resonant cavity. The plasma-filled tube acts as aconductor along which microwave energy travels out of the cavity, thuseffectively extending the plasma and reducing plasma density. Inaddition to reducing plasma density, and therefore reducing generationof radicals, the extension of the plasma also increases the risk of ionssurviving to reach the process chamber and substrate housed therein.Microwave traps can confine such microwave leakage. For example, U.S.Pat. No. 5,498,308 to Kamarehi et al., entitled “Plasma Asher withMicrowave Trap,” discloses a resonant circuit trap. Even employing suchtraps, however, the plasma expands outside the plasma source cavityalong the tube to the outer edges of the traps.

Accordingly, a need exists for more efficient microwave generators toimprove resist ash rates.

SUMMARY OF THE INVENTION

In satisfaction of this need, a remote plasma generator is provided forcoupling microwave frequency energy to a gas and delivering radicals toa downstream process chamber. The plasma generator includes severalfeatures which, in conjunction, enable highly efficient radicalgeneration and consequently high photoresist ash rate. Such high ashrates can be achieved for both standard photoresist and chemically moreproblematic residues, such as those created by ion implantation andreactive ion etching.

In accordance with one aspect of the invention, high power can becoupled to gas flow that includes a fluorinated chemistry. Asingle-crystal, one-piece sapphire applicator and transport tube resistsfluorine attack. The tube can be lengthened and provided with an elbowjoint, aiding recombination of ions and protecting the process chamberfrom UV radiation produced in the plasma discharge.

In accordance with another aspect of the invention, microwavetransparent cooling fluid enables high power, high temperature plasmaproduction. While useful for increasing practicable power forapplicators of any material, liquid cooling is of particular utility inconjunction with sapphire applicators, which are susceptible to stresscracking.

In accordance with still another aspect of the invention, dynamicimpedance matching via a sliding short at the terminus of a microwavecavity reduces power loss through reflected energy. At the same time, alow profile microwave trap produces a more dense plasma to increaseradical production.

In accordance with yet another aspect of the invention, differentradicals are separately generated and mixed just upstream of the processchamber, enabling individually optimized radical generation of the twospecies. In the illustrated embodiment, fluorine radicals are generatedin a sapphire applicator, while oxygen radicals are generated in aquartz applicator.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from thedetailed description below, and from the appended drawings, which areintended to illustrate and not to limit the invention, and wherein:

FIG. 1 is a schematic view of a semiconductor reactor incorporating aremote microwave plasma generator, constructed in accordance with apreferred embodiment of the present invention;

FIG. 2 is a right, front and top perspective view of a plasma generator,constructed in accordance with the preferred embodiment;

FIG. 3 is a left, front and top perspective view of the plasma generatorof FIG. 2;

FIG. 4 shows a subsystem of the plasma generator of FIG. 3;

FIG. 5 is a right, front and top perspective view of the subsystem ofFIG. 4, shown with a protective sheath removed from a carrier tube;

FIG. 6 is an exploded view of the subsystem of FIG. 4, taken from theangle of FIG. 5;

FIG. 7 is a side sectional view of the subsystem, taken along lines 7—7of FIG. 4;

FIG. 8 is a top down sectional view of the subsystem, taken along lines8—8 of FIG. 4;

FIG. 9 is a graph illustrating reflected power against microwavegenerator power, for both fixed tuning and dynamically or in-situ tunedimpedance matching;

FIG. 10 is a graph illustrating reflected power against total gas flow,for both fixed tuning and dynamically or in-situ tuned impedancematching;

FIG. 11 is a graph illustrating reflected power against gas pressure,for both fixed tuning and dynamically or in-situ tuned impedancematching;

FIG. 12 is a cross-sectional view of a component of the subsystem ofFIG. 4, including a low profile, co-axial microwave choke;

FIG. 13 is a rear, right perspective view of a semiconductor reactorincorporating dual plasma generators, constructed in accordance with asecond embodiment of the present invention;

FIG. 14 is a rear elevational view of the reactor of FIG. 13;

FIG. 15 is a cross-sectional view taken along lines 15—15 of FIG. 13;and

FIG. 16 is a pair of graphs illustrating ash rates and uniformity usingthe plasma generators of the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the illustrated embodiments are described in the context of aresist stripping or ashing system, the skilled artisan will readily findapplication for the devices and methods disclosed herein for othersystems. Within the semiconductor industry, for example, plasma or freeradical generation is desirable for assisting or enhancing many chemicaletch and chemical vapor deposition processes. For many of theseprocesses, remote production of plasmas advantageously avoids damage tothe substrate.

With reference initially to FIG. 1, a chemical downstream etch (CDE)reactor 10, according to a preferred embodiment, is schematicallyillustrated. The reactor 10 includes a microwave plasma generator 12upstream of a process chamber 14. A substrate 16, typically comprising amonolithic semiconductor wafer, is supported upon a chuck 18 over apedestal 20 within the chamber 14.

With reference to FIGS. 1-4, the plasma generator 12 includes amicrowave power source 22, which can be a conventional magnetron. Forexample, suitable microwave power sources are commercially availableunder the trade names N10230 and NL 10250 from Richardson Electronics ofLaFox, Ill. The NL10230 magnetron generator is capable of producingabout 3,000 W of microwave power at 2,450 MHz (nominal). The skilledartisan will readily appreciate that, in other arrangements, the powersource can be of any construction suitable for coupling power to a gas.Other plasma generators, for example, can employ radio frequency power,and energy can be coupled inductively or capacitively to the gas beingionized.

The illustrated plasma generator 12 further includes, adjacent the powersource 22, an isolator module 24, which can also be of conventionalconstruction. As is known in the field, the isolator 24 protects themagnetron from reflected power by diverting such reflected power to adummy load. Desirably, the isolator 24 includes an integrateddirectional coupler, which also measures reflected power in order tomatch impedance along the microwave energy pathway, as will be discussedin more detail with respect to the section entitled “ImpedanceMatching,” below.

Microwave power is directed through the isolator 24 to a waveguide 26,which extends into a microwave cavity or plasma source 28. As best seenfrom FIG. 2, the illustrated waveguide 26 is S-shaped, enabling astacked configuration and reducing reactor footprint on the fabricationfloor. The waveguide includes a fixed tuning knob 30 (FIG. 1), whichoperates in conjunction with an autotuner module 32 to match impedanceof the microwave energy path (including the isolator 24, waveguide 26and cavity 28) to that of the power source 22. The autotuner module 32is described in more detail in the section entitled “ImpedanceMatching,” below.

A gas carrier tube 34 extends from a gas source 36 through the microwavecavity 28. The tube 34 axis extends transversely to the waveguide axis.In the illustrated embodiment, the gas source includes an oxygen sourcegas (preferably O₂), a fluorine source gas (preferably CF₄ or NF₃), anda carrier gas (preferably N₂). As discussed in more detail in thesection entitled “Single-Crystal Transport Tube,” below, the carriertube 34 includes an upstream section 38, an applicator section 40, and atransport tube section 41. The transport tube 41 includes a bend orelbow joint 42 between the cavity 28 and the process chamber 14.

A pair of microwave emission barriers 44, 46, surrounding the applicator40 immediately upstream and downstream of the cavity 28, respectively,serve to prevent microwave energy escaping the cavity 28. Theconstruction of the emission barriers 44, 46 is discussed in more detailin the section entitled “Microwave Choke,” below.

Single-crystal Transport Tube

Referring to FIG. 1, as briefly noted above, the gas carrier tube 34includes three sections: the upstream section 38, carrying gas from thegas source 36; the applicator section 40, extending through themicrowave cavity 28; and the transport tube section 41, extendingdownstream of the cavity 28 to the process chamber 14.

In operation, microwave power conducted from the waveguide 26 is coupledto gas flowing through the applicator portion 40 of the tube 34 (withinthe cavity 28), exciting the gas and igniting a plasma. The applicator40, including portions of the tube 34 within the cavity 28 as well assections immediately adjacent the cavity 28, is directly subjected toenergetic particles of the plasma discharge and are consequently subjectto faster deterioration than upstream portions, and slightly faster thandownstream portions. Since the upstream section 38 of the tube 34 is notsubject to the plasma discharge and therefore does not deterioraterapidly, the upstream section 38 is preferably a conventional stainlesssteel gas line and is provided separately from applicator 40, such thatit need not be replaced when the applicator 40 is due for replacement.

In the illustrated embodiment, both fluorine and oxygen source gases areprovided to the applicator 40, generating O and F free radicals as wellas a variety of ionic species and electrons. As noted in the Backgroundsection above, fluorine is particularly corrosive to quartz tubing.Accordingly, the applicator 40 preferably comprises sapphire forresistance to fluorine attack. Most preferably, the applicator 40 isformed of single-crystal sapphire, providing superior physical strengthto withstand the stresses generated by exposure to the plasma.

As discussed in the Background section above, the length of thetransport section 41 of the carrier tube 34 is selected to allowrecombination of ions prior to introduction of the energized gas to theprocess chamber 14. Preferably, the transport section 41, from the endof the microwave cavity 28 to the process chamber 14, is at least about5 inches long, more preferably at least about 10 inches, and is, about14.5 inches in the illustrated embodiment. The total length of theapplicator 40 and transport section 41 is about 21.5 inches in theillustrated embodiment. The skilled artisan will understand, however,that shorter lengths of transport tubing can be used where ion contentis reduced in alternative manners.

The transport section 41 preferably includes the bend or elbow joint 42,best seen in FIG. 7, thus avoiding direct line of sight between the glowdischarge within the cavity 28 and the process chamber 14 (FIG. 1).Preferably, the bend 42 defines at least a 35° angle, and morepreferably greater than about a 45° angle. In the illustratedembodiment, the bend 42 defines a 90° or right angle. The substrate 16is thus shielded from harmful UV photons released by the glow discharge.

As the transport tube 41 is also subjected to energetic plasma products,including excited fluorine species, this section is also preferablyformed of sapphire, and more preferably single-crystal sapphire.Additionally, the transport tube 41 is preferably formed integrally withthe applicator tube 40.

Unfortunately, the crystalline quality of sapphire tends to degrade fortube lengths over 12 inches. As previously noted, the resultantpolycrystalline structure of longer lengths of sapphire tubing issusceptible to stress cracking when subjected to high power, hightemperature plasmas. The desirability of forming an elbow joint withinthe transport tube section 41 also necessitates joining at least twosections of single-crystal sapphire. Typical bonding materials, however,are incompatible with fluorine, such that employing such bonds wouldnegate the very advantage of sapphire tubing.

Accordingly, the illustrated transport tube 41 and applicator 40 areprovided as sections of single-crystal sapphire, bonded at the elbowjoint 42 without bonding materials susceptible to fluorine attack. Inparticular, the sections 40 and 41 are bonded by eutectic bonding, asdisclosed in PCT publication number WO 09 856 575, entitled EutecticBonding of Single Crystal Components, published Dec. 17, 1998 (the “PCTapplication”). The disclosure of the PCT '575 application publication isincorporated herein by reference. Single-crystal sections pre-bonded inthe manner of the PCT '575 application are available from Saphikon, Inc.of Milford, N.H.

The PCT '575 application describes a process for eutectically bondingtogether sections of single crystal-sapphire using a eutectic bondingmaterial that includes one or more Group IIIA compounds. Preferably, thebonding material includes an yttrium-containing compound, such asyttrium oxide, yttria (Y₂O₃), or yttrium aluminum garnet (YAG). Thesingle crystal sapphire sections are first etched to remove any surfacecontaminants and then coated with the bonding material. The sapphiresections are then assembled and heated to a temperature sufficient tomelt the eutectic bonding material but below the melting point of thesapphire sections. When the assembly is cooled, a high-strength eutecticbond is formed between the sapphire sections that is particularlyresistant to fluorine attack.

Applicator Cooling System

The employment of single-crystal sapphire provides resistance tofluorine attack and greater strength than polycrystalline sapphire.Accordingly, the single-crystal sapphire tube, serving as an integralapplicator 40 and transport tube 41, enables coupling relatively highpower to the gas, while withstanding fluorine attack for applicationssuch as post-implantation ashing.

Coupling high power to the gas desirably increases the rate of O and/orF radical production, thereby increasing ash rates. However, the kineticenergy created in a high power plasma introduces negative effects aswell. Within the cavity 28, rapid and frequent collisions betweenenergized particles, and between such particles and the applicatorwalls, raises the temperature of the applicator 40, creating thermalstresses on the tube. While single-crystal sapphire is more resistant tosuch stresses than polycrystalline sapphire, sapphire remains subject tostress cracking under high power, high temperature operation, ascompared to quartz. Moreover, the high temperature of the applicator 40encourages recombination of dissociated particles. While recombinationof ions and electrons is desirable, recombination of free radicals (F,O) is counterproductive.

Accordingly, the preferred embodiments employ a cooling mechanism withinthe applicator 40, thereby reducing kinetically-induced recombinationwithin the applicator 40. Lengthening the transport tube 41 compensatesfor reduced ion recombination within the applicator 40. Energetic ionsrecombine over the length of the transport tube 41 in greaterproportions than radicals, due to additional electrostatic reactionsencouraging such recombination. At the same time, cooling the applicator40 enables use of higher power for a given tolerance for thermal stress.For sapphire applicators, in particular, the cooling mechanism reducesthe occurrence of stress cracking while boosting the efficiency of freeradical generation.

With reference to FIGS. 7 and 8, a cooling jacket 50 surrounds theapplicator section 50 of the carrier tube 34. The space between thejacket 50 and the applicator 40 is filled with a coolant fluid.Advantageously, the fluid is circulated through the jacket 50, enteringat a fluid inlet 52 (FIG. 7) and exiting at a fluid outlet 54 (FIG. 8).The inlet 52 and outlet 54 are arranged at 90° to one another, toencourage circumferential circulation of the fluid. The cooling jacket50 preferably extends upstream and downstream of the microwave cavity28, as shown.

Desirably, both the jacket 50 and the cooling fluid comprise microwavetransparent materials, thus maximizing microwave energy coupling to thegas within the applicator 40, rather than direct absorption by thejacket 50 and coolant. The cooling jacket 50 preferably comprisesquartz.

The coolant fluid is selected to have minimal hydrogen (H) content,which readily absorb microwave energy. Preferably, the coolant containsno hydrogen, and in the illustrated embodiment comprises aperfluorinated, inert heat transfer fluid. Such fluids are availablefrom the Kurt J. Lesker Company of Clairton, Pa. under the trade nameGalden™. Advantageously, this liquid coolant is available in multipleformulations having different boiling points. Thus, the most appropriateformulation can be selected for cooling the microwave applicator,depending upon the desired parameters for operating the microwave plasmagenerator.

Accordingly, recombination of desirable radicals within the applicator40 is reduced by provision of liquid cooling of the applicator 40.Moreover, greater power can be coupled to the gas without damage to theapplicator 40. In the illustrated embodiment, the power source 22 can beoperated at full power (about 3,000 W) under normal operating conditions(i.e., continuous or intermittent operation while sequentially ashingphotoresist from multiple wafers), without inducing stress cracks in thesapphire applicator 40. It will be understood that improved powertolerance, and hence more efficient radical production, are alsoapplicable to quartz applicators, which are generally more desirable fornon-fluorinated chemistries. It will be understood that operable powerlevels may be considerably higher for quartz applicators with liquidcooling.

To a lesser extent, the downstream transport tube 41 is also heated byexposure to plasma by-products. The downstream tube, however, is notdirectly contacted by the glow discharge. Rather than liquid cooling thetransport tube 41, therefore, the preferred embodiments provide aninsulated shroud 56 around the tube, as shown in FIGS. 2-4 and 6,reducing risk of burns to technicians. Preferably, fans direct airthrough the insulated shroud 56, cooling the transport tube 41 byconvection.

Impedance Matching

With reference to FIGS. 1-3, the microwave energy generated by the powersource 22 is propagated through an energy path including the isolator24, waveguide 26 and microwave cavity 28. The impedance of the varioussections of the energy path should be closely matched to avoid energyloss through reflected power. By careful impedance matching, a standingwave or resonant condition is created in the microwave waveguide system26, 28, where the power is coupled to gas flowing through the applicator40. While the power source is protected from reflected energy by theisolator 24, reflected energy absorbed by the dummy load in the isolator24 represents wasted power that would otherwise be available for radicalgeneration.

Impedance matching is complicated by the fact that the medium withinwhich the microwaves propagate is of variable composition. The densityand conductivity of gas flowing through the microwave cavity 28 varieswith different process recipes. Since a reactor is typically utilizedrepetitively for the same recipe by a semiconductor manufacturer,impedance matching is typically performed for a given process recipe.Tuning the impedance of the waveguide 26 (including the cavity 28) isthus necessary when the reactor is first shipped to the manufacturer, aswell as each time the process recipe is changed.

A common method of tuning impedance of the waveguide 26 is by employingthree tuning knobs within the waveguide 26. By adjusting the amount ofprotrusion of these conductors tranversely across the waveguide 26 atthree different locations along the energy propagation axis, impedanceof the waveguide 26 can be matched to that of the isolator 24 and powersource 22, thus minimizing reflected power for a given process recipe.This manner of impedance matching is known as a triple stub tuner. Whileeffective in minimizing reflected power, triple stub tuners areexpensive.

In the preferred embodiments, however, impedance matching is controlledby the combination of a fixed tuning knob 30 (FIG. 1) within thewaveguide 26 and a sliding short 60, shown in FIG. 8. The tuning knob 30is preferably factory preset for gross tuning, while the sliding short60 dynamically fine tunes the impedance matching.

The sliding short 60 is driven by a motor actuator 62 within theautotuner module 32. The sliding short 60 comprises a conductor whichextends across the walls of the microwave cavity 28, thus providing amovable end wall for the cavity 28. The position of the sliding short 60can be changed until impedance of the waveguide 26 (including themicrowave cavity 28) closely matches that of the isolator 24, at whichpoint a resonant condition is achieved within the cavity 28. Moreover,adjustment of the sliding short 60 along the energy propagation axis, inplace of transverse movement of a triple stub tuner in the waveguide 26,facilitates arranging the standing wave pattern to optimize coupling ofenergy to the gases flowing through the applicator 40.

Moreover, as the name implies, the autotuner 32 matches impedancedynamically via closed loop control. Reflected power is continuallymeasured at the isolator module 24 (FIGS. 1-3) and sends signals to anelectronic controller (not shown). The controller, in turn, sendssignals to the motor actuator 62, which drives the sliding short 60.After the sliding short 60 moves, the change in reflected power isrecognized by the controller, which then further adjusts the position ofthe sliding short 60, and so on until reflected power is minimized.

In the illustrated embodiment, the sliding short 60 is initiallypositioned one half of the microwave energy wavelength (as measured inthe waveguide system) from the center of the applicator 40 runningthrough the cavity 28. In this position, the magnetic field is maximum,and a generally low electric field strength reduces damage from ionacceleration downstream. If necessary, the short 60 can be moved to onequarter of the microwave energy wavelength for initial ignition and thenmoved to one half wavelength after the plasma is essentiallyself-supporting. The motor actuator 62 preferably enables deviationforward or back from this default position by up to about 0.25 inch,more preferably up to about 0.5 inch, and in the illustrated embodiment,the sliding short 60 is movable by about 0.75 inch to either side of thedefault (½ wavelength) position. In another embodiment, the actuator 62enables deviation from the default position of up to a half wavelengthin either direction (±1.7 inches), for fully adjustable impedancematching. The autotuner module 32 further includes optical sensors toprevent over-movement of the sliding short 60. It has been found that,not only does the above-described arrangement (short 60 at λ/2 fromapplicator center) minimize reflected power, but it also maximizesmicrowave magnetic field intensity within the applicator.

Dynamic closed loop control of impedance matching thus accommodatesfluctuations in parameters in operation of a single process recipe.Additionally, autotuning can accommodate various process recipes. FIGS.9-11, for example, illustrate the effect of reflected power againstvarious process parameters, including differences in power sourceoutput, total gas flow and gas pressure. It will be understood thatother process parameters, such as constituent gas makeup, will alsoaffect reflected power. As illustrated, dynamic or in-situ impedancematching reduces losses from reflected power, relative to fixed tuning,without the downtime required for manually tuning for each differentprocess recipe. Such autotuning is particularly advantageous forresearch and development of new processes, where it is desirable to testmany different processes for optimization.

Microwave Choke

With reference to FIG. 1, while the autotuner module 32 minimizes thepower reflected back toward the power source 22, impedance matching doesnot address the possibility of microwave leakage through the openings inthe cavity 28 through which the applicator 40 extends upstream anddownstream. Such leakage is disadvantageous for a variety of reasonsdiscussed in the Background section above, and in U.S. Pat. No.5,498,308 to Kamarehi et al., entitled “Plasma Asher with MicrowaveTrap” (hereinafter “the '308 patent”). The disclosure of the '308 patentis incorporated herein by reference.

In order to minimize this leakage, therefore, the preferred embodimentsare provided with the upstream microwave choke or emission barrier 44and the downstream microwave choke or emission barrier 46. FIGS. 4-8show these emission barriers 44, 46 in relation to the microwave cavity28 and the carrier tube 34, specifically in relation to the applicatorsection 40 of the carrier tube 34.

FIG. 12 shows the emission barrier 46 in isolation. As illustrated, theemission barrier 46 comprises an inner conductor 70, an outer conductor72, and a dielectric medium 74. Each of these components is rectangular(see the exploded view of FIG. 6) and surrounds the applicator 40 justoutside the microwave cavity 28. When assembled, the inner and outerconductors 70, 72 define a choke cavity filled with the dielectricmedium 74, having a gap or opening 75 between the inner and outerconductors 70, 72 at a distal end of the choke cavity.

The inner conductor 70 and the outer conductor 72 define co-axialconductors selected to have an electrical length of a quarter of awavelength of the microwave s energy of interest. As disclosed, forexample, in “Fields and Waves in Communication Electronics,” Ramo,Whinnery and Van Duzer, p. 46, Table 1.23 (hereinafter, “Ramo et al.”)the impedance of an ideal quarter wavelength line is given by thefollowing formula, $Z = \frac{Z_{0}^{2}}{Z_{L}}$

where Z is the impedance of the coaxial line, Z₀ represents thecharacteristic impedance of the medium through which the electromagneticwaves travel and Z_(L) represents the load impedance. In the illustratedembodiment, the coaxial line is shorted across the inner conductor 70and outer conductor 72 at the proximal end 76. As the load impedanceZ_(L) of a shorted line is ideally zero, the microwave energypropagating from the microwave cavity 28 toward the distal end of thechoke 46 meets with an impedance approaching infinity at the opening 75of the choke cavity, regardless of the characteristic impedance Z₀.

The high impedance of an open ended, shorted, quarter wave coaxial linecan alternatively be shown by using the formula for the impedance of ashorted line, as also disclosed in Ramo et al.:

Z=jZ _(o)tan(βl)

The phase constant β equals 2π/λ, while the length of the line has beenselected to be a quarter wave, or λ/4. Inserting these values into theformula above, the tangent term (tanβl) becomes the tangent of π/2,which approaches infinity.

Referring again to FIG. 1, while the high impedance microwave chokes 44,46 limit leakage of microwaves past the opening 75 of the choke cavity,the energy still propagates along the inner conductor 70 to the opening75 of the choke. Energy thus continues to couple to the gas within theapplicator 40 to this point, expanding the plasma beyond the confines ofthe microwave cavity 28, both upstream and downstream of the cavity 28.Such plasma expansion is disadvantageous for a number of reasons. Asnoted above, the expansion of plasma downstream of the cavity 28increases the likelihood that energetic ions and/or UV radiation fromplasma glow discharge will reach the process chamber 14. Moreover, theexpansion of the plasma disadvantageously reduces plasma density. Aswill be recognized by the skilled artisan, increasing the plasma densityfacilitates more efficient generation of free radicals for a given powerinput.

Accordingly, the dielectric medium 74 is selected to have a highdielectric constant. In contrast to air (dielectric constant=1), theillustrated dielectric medium 74 preferably comprises a solid materialhaving a dielectric constant of at least about 3.0, more preferablygreater than about 5, and comprises about 9 in the illustratedembodiment. The exemplary material of the illustrated embodimentcomprises a ceramic, more particularly Stycast™ Hi K, available fromEmerson & Cuming.

Microwaves travel on the surface of conductors within the highdielectric medium 74, i.e., along the interior of the choke cavity.Thus, the absolute distance of a quarter wave in the exemplary ceramicis much shorter than the absolute distance of a quarter wave in air(about 1.2 inches for 2,450 MHz microwave energy). In the illustratedembodiment, a quarter wave through the ceramic translates to an absolutedistance of about 0.4 inch, since the quarter wave length isproportional to the square root of the medium's dielectric constant.

The effective volume of the plasma generated within the cavity 28 andthe leakage out to the opening of the choke cavity is thus reduced withincreasing dielectric constant of the dielectric medium 74.Consequently, the density of the plasma is improved for a given powerinput, and the efficiency of radical generation improves. Improvedradical generation, in turn, results in increased ash rates in theillustrated plasma ash reactor.

Segregated Plasma Sources

FIGS. 13-15 illustrate a plasma ash reactor 100 constructed inaccordance with a second embodiment of the invention. It will beunderstand that the reactor 100 preferably includes one or more, andmore preferably all of the above-noted features of the invention. As thereactor 100 includes many features which can be similar or identical tothose of the previously discussed embodiments, like features will bereferred to by like reference numerals, with the addition of the number100.

The illustrated reactor includes a first process chamber 114 and asecond process chamber 115. As the two chamber 114 and 115 can haveidentical construction, the present description will focus on the firstchamber 114 and the plasma generators associated therewith.

The chamber 114 has two plasma generators 112 and 112′, each of whichcan have a similar construction as that disclosed above (with particulardistinctions noted below). Each of the two plasma generators 112, 112′leads generated radicals, via transport tubes 141, 141′ to the firstprocess chamber 114, as shown. The free radicals from each generator aremixed, prior to introduction to the process chamber 114, in a mixerchamber 145.

With reference to FIGS. 13 and 14, it can be seen that the microwavecavities 128 and 128′ of the two generators 112 and 112′ are transverseto one another. This arrangement enables closer packing of the moduleswithin the reactor frame, saving footprint on the clean room floor.

With reference to FIGS. 14 and 15, the transport tubes 141, 141′ eachcommunicate with the mixer chamber 145 via injectors 147, 147′.Advantageously, the injectors 147, 147′ are configured to injectradicals tangentially near the circumference of the mixer chamber 145,thus facilitating mixing of the radicals from the two different sources128, 128′. Most preferably, the injectors 147, 147′ inject with oppositeorientation, such as clockwise and counter-clockwise, creatingturbulence and aiding the mixture of reactive species from each of theplasma sources 128, 128′.

The interior walls of the mixer chamber 145 and the injectors 147, 147′preferably comprise anodized aluminum, but can also be fabricated ofpolished sapphire for improved surface smoothness. In either case, thechamber 145 preferably has the same chemical makeup as sapphire (Al₂O₃),which is advantageously resistant to fluorine attack. The mixer chamber145 has a low profile, preferably less than about 1.0 inch in height,more preferably less than about 0.5 inch, and is about 0.22 inch high inthe illustrated embodiment. The low profile presents less wall surfaceto the radicals, and thus reduced recombination of free radicals.

As shown in FIG. 15, mixer chamber 145 includes a relatively smallcentral window 149 in the floor, opening into a first plenum chamber 153defined above a first perforated baffle plate 151. A second baffle plate155 below the first baffle plate 151 defines a second plenum chamber 157between the baffle plates 151, 155. The second baffle plate 153 includesperforations (not shown) which are misaligned relative to theperforations of the first baffle plate 151. Together, the baffle plates151, 155 ensure uniform delivery of free radicals to the process chamber114 below.

The separate plasma generators 112, 112′ advantageously enableindividual optimization for different reactants. For example, thematerial used for the applicator and transport tubes have individualadvantages and disadvantages, which tend to favor one material for acertain process recipe and another material for another process recipe.As noted above, sapphire advantageously exhibits resistance to fluorineattack. The disadvantage of sapphire, however, is that it exhibitsundesirable recombination of free radicals. The table below illustrates,by way of example, the recombination coefficient (γ) of variousmaterials.

Material Recombination efficiency (γ) silver 1.000 copper 0.708 iron0.150 nickel 0.117 aluminum oxide (sapphire) 0.009 glass (quartz) 0.001Teflon ™ 0.0001

The above table illustrates that sapphire exhibits about nine times morerecombination of desirable radicals than quartz. Thus, while desirablefor resisting fluorine attack, sapphire significantly reduces theefficiency of radical delivery to the process chamber.

Thus, in the illustrated dual plasma source reactor 100, the firstplasma generator 112 includes a single-crystal sapphire, integralapplicator 140 and transport tube 141, and is optimized for fluorineradical generation. Thus, the sapphire tube withstands fluorine attack.The second plasma generator 112′ includes a quartz applicator 140′ andtransport tube 141′, and is optimized for oxygen radical generation.Thus, the quartz tube minimizes undesirable recombination of oxygenradicals, relative to sapphire, and at the same time can withstandhigher power and higher temperature plasma production than sapphire.

Accordingly, oxygen radical production is not subjected to thelimitations on plasma generation imposed by fluorine corrosion, whileboth fluorine and oxygen radicals are introduced to the processingchamber 114. Moreover, if desirable for ashing photoresist at back-endstages of semiconductor fabrication, where the resist has not beenimplanted with ions, the fluorine plasma generator 112 can be turnedoff, and only the oxygen plasma generator 112′ operated. Similarly,where only fluorine etching is desired, the oxygen plasma generator 112′can be turned off.

The skilled artisan will readily appreciate, in view of the presentdisclosure, application for the dual plasma source reactor for otherplasma- or radical-assisted processes. In such other arrangements, itmay be desirable to independently optimize features other than thematerial of the applicator tube, the source gas and the power level.

FIG. 16 illustrates very high plasma ash rates and uniformity obtainedwith the features described above. As shown, the preferred embodimentscan achieve ash rates greater than about 6 μm/min, with a Shipleyresist, with about 2% non-uniformity or less, and ash rates greater thanabout 8 μm/min with a Sumitomo resist with non-uniformity of about 2.5%or less.

Although the foregoing invention has been described in terms of certainpreferred embodiments, other embodiments will become apparent to thoseof ordinary skill in the art in view of the disclosure herein.Accordingly, the present invention is not intended to be limited by therecitation of preferred embodiments, but is intended to be definedsolely by reference to the appended claims.

We claim:
 1. A plasma generator comprising a hollow sapphire tube extending from a gas source through a microwave cavity to a process chamber, the tube including a sapphire elbow joint defining an angle of greater than about 35° between the microwave cavity and the process chamber.
 2. The plasma generator of claim 1, further comprising a microwave power source coupled to the microwave cavity.
 3. The plasma generator of claim 2, wherein the microwave power source can couple at least about 3,000 W of microwave energy at about 2,450 MHz to gas within the sapphire tube.
 4. The plasma generator of claim 2, wherein a microwave energy pathway from the generator, includes: an isolator module in communication with the microwave power source, the isolator module configured to protect the power source from reflected power; and a waveguide communicating at a proximal end with the isolator module and communication at a distal end with a proximal end of the microwave cavity, wherein the cavity includes a gas influent port and a radical effluent port and a sliding short defining a variable distal end of the microwave cavity, the sliding short dynamically controlled to match impedance of the microwave cavity with the waveguide.
 5. The plasma generator of claim 4, wherein the microwave energy pathway includes a directional coupler measuring reflected energy directed toward the microwave power source, the directional coupler generating signals controlling movement of the sliding short.
 6. The plasma generator of claim 4, wherein preset tuning is conducted via a fixed tuning knob within the waveguide and fine tuning is conducted dynamically by the sliding short.
 7. The plasma generator of claim 2, further comprising a microwave choke including quarter-wavelength shorted coaxial conductors, the shorted coaxial conductors defining a choke enclosure surrounding the sapphire tube at an edge of the microwave cavity, the enclosure filled with a solid material having a dielectric constant greater than about
 3. 8. The plasma generator of claim 7, wherein the solid material comprises a ceramic.
 9. The plasma generator of claim 7, wherein the solid material has a dielectric constant greater than about
 5. 10. The plasma generator of claim 9, wherein the solid material has a dielectric constant of about
 9. 11. The plasma generator of claim 1, wherein the gas source comprises fluorine.
 12. The plasma generator of claim 1, herein the elbow joint defines an angle of about 90°.
 13. The plasma generator of claim 1, further comprising a cooling jacket surrounding the sapphire tube within the cavity, the cooling jacket filled with a perfluorinated cooling fluid transparent to microwave energy.
 14. The plasma generator of claim 13, wherein the cooling fluid contains no hydrogen.
 15. The plasma generator of claim 11, wherein the tube is in fluid communication downstream of the elbow joint with a plasma mixer chamber, the plasma mixer chamber also in fluid communication with a second gas carrier tube downstream of a second microwave cavity, the mixer chamber being upstream of a process chamber.
 16. The plasma generator of claim 15, further comprising a first perforated baffle plate positioned between the process chamber and the mixer chamber.
 17. The plasma generator of claim 16, further comprising a second perforated baffle plate positioned between the process chamber and the mixer chamber, wherein the first and second baffle plates have non-aligned perforations.
 18. The reactor of claim 15, wherein the second gas carrier tube comprises quartz.
 19. The reactor of claim 18, wherein the sapphire tube communicates with a source of fluorine and the second gas carrier tube communicates with a source of oxygen.
 20. A plasma generator comprising a hollow sapphire tube extending from a gas source through a microwave cavity to a process chamber, the tube including an elbow joint defining an angle of greater than about 35° between the microwave cavity and the process chamber, the elbow joint comprising at least two single crystal sapphire elements bonded together with a eutectic bonding material.
 21. The plasma generator of claim 20, herein the eutectic bonding material comprises one or more Group IIIA compounds.
 22. The plasma generator of claim 20, wherein the eutectic bonding material comprises an yttrium-containing compound, such as yttrium oxide, yttria (Y₂O₃), or yttrium aluminum garnet (YAG). 